This invention relates to systems for carbon black production, and more particularly, to systems for producing carbon black of varying properties.
Carbon black reactors and methods of producing carbon black are well known in the prior art. Most modern carbon black reactors comprise a longitudinally extending reactor tunnel of refractory material of generally circular cross-sectional configuration defining combustion, reaction, and quench zones in contiguous axial alignment. A hydrocarbon fluid fuel, usually natural gas or fuel oil, is burned in the combustion zone with a stream of process air furnished by a blower. The hot gases produced by the combustion of the fuel are directed into the reaction zone of the reactor tunnel and admixed with injected carbon black feedstock, usually a heavy aromatic oil. As the oil enters the flowing hot gases, it evaporates and undergoes a series of reactions to produce the product carbon black. The reacting carbon black and hot gases are quenched, usually by water spray, in the quench zone to a temperature below that required for reaction. The produced carbon black and off gases are collected and separated.
The physical and chemical changes that occur in the formation of carbon black from the evaporated feedstock oil are very complex. Heat is transferred rapidly to the oil vapor from the hot combustion gases, hot refractory, and combustion of a portion of the oil by residual oxygen. Under these conditions, feedstock oil molecules are dehydrogenated forming carbon nuclei. The nuclei grow in size to form particles which aggregate into cluster-like agglomerates, commonly known as "structure". The particles continue to grow in size after the "structure" is formed. These two properties, "structure" and particle size, are of a paramount importance in the production of carbon black since they determine to a large extent the properties of manufactured articles containing carbon black. Carbon blacks of high "structure" level and small particle size are particularly valuable since they impart increased resistance to abrasion to rubber products.
The particle size and "structure" level of produced carbon black may be measured by a variety of tests. Particle size is normally determined by measuring the surface area of the produced carbon black particles. Surface area may be determined directly or indirectly. The "tint" test, an indirect determination, measures the ability of a produced carbon black to cover the surface of a finely divided zinc pigment as compared with a standard carbon black of known particle size. Adsorptive tests are used to directly measure the surface area and thus particle size of produced carbon black. Included among these absorptive tests are the nitrogen adsorption test, the iodine test, and the cetylmethyl ammonium bromide (CTAB) test. The CTAB molecules are too large to enter pores in the carbon black particles that may be entered by smaller molecules such as nitrogen. The difference between the CTAB test results and the nitrogen adsorptive test results is a measure of the porosity of the carbon black particles, another important property. "Structure" level is measured by the amount of dibutyl phthalate absorbed by a given sample of produced carbon black. The sample of carbon black may be tested for DBP adsorption both before and after being subjected to an exact crushing pressure, since the "structure" level of produced carbon black drops to a lower constant value upon mechanical working, such as would be encountered by mixing carbon black with other raw materials to produce manufactured articles. The test is known as the DBP test when it is performed without crushing the carbon black. When the carbon black is crushed before absorption, the test is known as the "24M4 DBP" test since the sample is subjected to 24,000 p.s.i. of pressure four times before the dibutyl phthalate is added. A direct test of the properties of produced carbon black may be carried out by compounding the carbon black into manufactured articles which are then subjected to tests. Representative of these tests is the "road test". Samples of produced carbon black are compounded into a standard rubber formulation, made into tire treads, and tested for abrasion resistance on the road. A standard carbon black is tested to give a reference value arbitrarily set at 100. Values greater than 100 indicate superior resistance to abrasion and values less than 100 indicate inferior resistance.
It is well known in the art that smaller particle size can be achieved by reacting the feedstock oil in an area of the reaction zone where the flowing hot gases have an increased velocity. It is well known in the art that such increased velocity of the flowing hot gases can be achieved through a constriction in the reactor tunnel. Venturi constrictions are among the configurations of constrictions employed. Nevertheless, it is also known that "structure" formation is favored by higher pressures in the reaction zone. Since the constrictions employed to increase velocity have the effect of lowering pressure, smaller particle size is normally obtained at the expense of "structure". Thus, in the prior art, carbon black reactors particle size and "structure" level were balanced against each other to provide an acceptable carbon black product. Representative of prior art carbon black systems employing one venturi or other constriction in the reaction zone are U.S. Pat. Nos. Cheng 3,877,876; Latham 3,256,065; Heller 2,851,337; and Heller 3,490,869. Representative of prior art carbon black systems using the two nonventuri constrictions are U.S. Pats. No. Sweigert 2,682,450; Hess 3,222,136; and Schirmer 3,248,252. None of the references show or suggest the use of two venturi constrictions in the reaction zone to create novel pressure profiles therein.
A further problem encountered with prior art carbon black reactors is that a variety of carbon blacks of different properties are desired. Each prior art carbon black reactor can be adjusted to produce only four or five grades of carbon black out of the seventy or more grades that are recognized. Adjustment in the prior art carbon black reactors to produce different grades of carbon black is commonly done by changing such factors as velocity, air to fuel ratios, air to feedstock ratios, position and direction of the feedstock injection, and quench position. Grades of carbon black are normally determined by particle size, "structure" level, porosity, and degree of oxidation of the particle surface. Most lower grades of carbon black have large particle size and low "structure" level. Higher grades have small particle size and high "structure" level. Within a given particle size and structure range, the degree of oxidation of the surface and the degree of microporosity may vary; these properties should be controlled. Some grades have a porous surface and others do not. Thus, the problems of the prior art devices are not only the difficulty of producing small particle size, high "structure" level carbon blacks but also the limitation as to the number of grades of carbon black of different properties that can be produced in a single carbon black reactor.